The mother and son accused of killing an Upper East Side woman ultimately failed in what prosecutors charge was an attempt to steal the socialite's mansion, but last week they were said to have pulled off a theft almost as difficult: conning an auto from a used-car dealer.

The pair, Sante and Kenneth Kimes, are on trial in State Supreme Court in Manhattan, charged with murdering the woman, Irene Silverman, and with 79 other crimes, including the theft of a green 1997 Lincoln Town Car from Parkway Motors in Cedar City, Utah.

The car is important to prosecutors because they believe the Kimeses used it to dispose of Mrs. Silverman's body, which has never been found. In addition, the police were led to arrest the Kimeses on July 5, 1998, hours after Mrs. Silverman was last seen, because of the car, which authorities said the Kimeses had bought with a bad check.

Last week, James A. Blackner, the sales manager for the car dealership, testified that he agreed to sell the Lincoln to Mrs. Kimes in February 1998. An employee delivered the car to Los Angeles, where the Kimeses were staying, and returned with a check for $14,973.50 and Mrs. Kimes's 1993 Lincoln as a trade-in.

''A 5-year-old child can see we didn't steal any car,'' she said. ''In fact, they took our car, they tricked us. We are going to sue them.''

A Little Word Play

The judge in the case, Justice Rena K. Uviller, has made it plain that she will not tolerate exaggerated language from the defense. But Mel A. Sachs, a lawyer for Mr. Kimes, managed to sneak in a pun after prosecutors spent almost an hour questioning Ramon Casales, the superintendent of Mrs. Silverman's mansion, about her keys. They were found in the possession of Mr. Kimes, who rented an apartment in the town house.

''You know a lot about the keys in the building,'' Mr. Sachs said to Mr. Casales. ''It's fair to say you are the prosecution's key witness.''

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The lead prosecutor's objection was lost in the chuckles and groans of jurors.

Puns aside, prosecutors did elicit important testimony from the witness. Mr. Casales said that after the authorities started looking for Mrs. Silverman, he searched the entire house, finishing in Mr. Kimes's apartment, where he found rolls of duct tape and large garbage bags. Investigators believe the Kimeses used the bags and tape to dispose of Mrs. Silverman's body.

And Mr. Sachs, in his cross-examination, inadvertently allowed the witness to make explicit what prosecutors had left implicit. ''You went to all the other apartments first, so this was the least suspicious of the apartments?'' Mr. Sachs asked.

Mr. Casales shook his head and, speaking through a Spanish translator, replied, ''No. On the contrary, the most suspicious.''

A Number Withheld

Prosecutors also had Mr. Casales identify Mrs. Silverman's voice when they played a tape of a telephone conversation allegedly recorded by the Kimeses, who are charged with tapping her phone.

On the tape, a caller, who prosecutors believe is Mrs. Kimes, offers Mrs. Silverman a free trip to Las Vegas if she will divulge her Social Security number. ''I have to have the number to send you the information,'' the caller says.

Mrs. Silverman, having already said she does not remember her number, strikes back quickly: ''Well, you send me whatever you have there, and if I like it, the idea, I will give you my Social.'' Prosecutors say the Kimeses needed the number to complete a fraudulent deed and steal Mrs. Silverman's house.